Flood, levee, and erosion control glossary

To help site visitors understand flood fighting, flood control, flood protection, and erosion terms and terms associated with coastal and inland waters, Infrastructure Defense Technologies has included this one-of-a-kind glossary. The glossary is organized as an alpha listing to enable visitors to quickly find the term being looked for.

Numeric | A | B | C | D | E | F | G | H | I | J | K
L | M | N | O | P | Q | R | S | T | U | V | W | XYZ


1% Flood

Refers to a flood of a magnitude that has an estimated probability of 1 in 100 of occurring in any given year. Technically more precise way of referring to the "100-year flood". Generally, 1%, 2%, 10% events refer to levels of flood flows with an expected recurrence of 100, 50, and 10 years respectively.

1-Percent-Annual-Chance Floodplain

An area that has at least a 1% annual chance of flooding and is designated as a Special Flood Hazard Area (SFHA) on the Flood Insurance Rate Map.

100-Year Flood

A term commonly used to refer to the one percent annual chance flood. The "100-year" flood is the flood that is equaled or exceeded once in 100 years on the average, but the term should not be taken literally as there is no guarantee that the "100-year" flood will occur at all within the 100-year period or that it will not recur.

100-Year Frequency Flood

A flood having an average frequency of occurrence on the order of once in 100 years although the flood may occur in any year (a one percent chance of being exceeded in any year). It is based on statistical analyses of streamflow records available for the watershed and analyses of rainfall and runoff characteristics in the general region of the watershed and is a statistical means of estimating the probability of flooding for insurance and land use planning. Over the life of a 30-year mortgage, there is approximately a 25- percent chance that this flood or one of a greater magnitude will occur. For these and NFIP regulations the base flood is the 100-year frequency flood.


A Unified National Program for Floodplain Management

A concept (and document of the same name) establishing the foundation of a coordinated national effort to manage the Nation's floodplains. The Unified National Program recommends a continuing unified program for planning and action at all levels of government to reduce the risk of flood losses and to protect floodplain values. The document was originally prepared by the U.S. Water Resources Council in 1976 in response to a directive of the 1968 National Flood Insurance Act, and subsequently revised and updated in 1979 and 1986.


That portion of the coastal floodplain as marked on maps prepared by the Federal Emergency management Agency that is likely to be inundated by the one-percent ("100-year") flood and not subject to wave action. The A-zone, however, may be subject to residual forward momentum of breaking waves. The A-zone and the V-zone together form the Coastal Special Flood Hazard Area.

Abandoned Well

A well that has been permanently disconnected and filled. Most States have regulations or guidelines for abandoned wells to ensure that contamination cannot move from the surface into the aquifer.


Removal of streambank soil as a result of sediment-laden water, ice, or debris rubbing against the bank.


The foundation along the sides of the valley or gorge against which the dam is constructed.

Accelerated Erosion

Erosion much more rapid than normal, natural, or geologic erosion, primarily as a result of the influence of the activities of man or in some cases, of other animals or natural KD that expose base surfaces, for example, fires or flooding.

Accelerated erosion - photo
Photo courtesy of USDA NRCS.

Accelerated erosion along a river bank.

Accredited Levee

A levee that has been documented to provide protection from the 1%-annual-chance flood. Areas landward of accredited levees are shown on FEMA’s flood map as Zone X (shaded), which denotes an area of moderate flood risk. Exceptions are areas of residual flooding, which are shown as Special Flood Hazard Area - SFHAs.

Acid Deposition

A complex chemical and atmospheric phenomenon that occurs when emissions of sulfur and nitrogen compounds and other substances are transformed by chemical processes in the atmosphere, often far from the original sources, and then deposited on earth in either wet or dry form. The wet forms, popularly called "acid rain," can fall as rain, snow, or fog. The dry forms are acidic gases or particulates.

Acid Neutralization Sump

A basin in which wastewater containing acids is neutralized prior to being discharged to a sewer.

Acid Soil

Soil with a low pH (below 7.0).


A volume of water equal to one acre covered to a depth of one foot; often used to describe the volume of a reservoir. One acre foot equals 43,560 cubic feet, or 326,700 gallons.

Active Floodplain

The level area adjacent to a stream channel that is subject to frequent overflow.

Actuarial Rates

Insurance rates determined on the basis of a statistical calculation of the probability that a certain event will occur. Actuarial rates are also called "risk premium rates." They are established by the Federal Insurance Administration pursuant to individual community Flood Insurance Studies and investigations that are undertaken to provide flood insurance in accordance with the National Flood Insurance Act and with accepted actuarial principles, including provisions for operating costs and allowances.

Acute Toxicity

Any poisonous effect produced within a short time after exposure to a toxic compound, usually within 24 to 96 hours.

Adjacent Steep Slope

A slope with a gradient of 15 percent or steeper within 500 feet of the site.

Adjustable Groin

A groin whose permeability can be changed, usually with gates or removable sections.


Removal of a pollutant from air or water by collecting the pollutant on the surface of a solid material; e.g., an advanced method of treating waste in which activated carbon removes organic matter from waste-water

Advisory Level

A level of chemical concentration in fish or shellfish whereby consumption of the fish would pose a human health risk. Levels may be determined by various federal or state agencies and may lead to advisories such as restricted consumption or consumption bans. Typical chemicals for which advisories exist include PCBs, chlordane, and dioxins.


A process that promotes breakdown of contaminants in soil or water by exposing them to air.


A mechanical process used to cultivate the soil without destruction of the turf


The appeal or beauty of objects, animals, plants, scenes, and natural or improved areas to the viewer and the appreciation for such items

Affected Public

1.The people who live and/or work near a hazardous waste site. 2. The human population adversely impacted following exposure to a toxic pollutant in food, water, air, or soil.

Age, Wave

The ratio of wave velocity to wind velocity (in wave forecasting theory).


Crushed rock or gravel screened to different sizes for various uses in construction projects.

Agricultural Lands

Fields, pastures, and orchards that are managed to produce food and fiber for people. Agricultural lands provide a source of income and employment, open space for recreational activities, and important habitat for wildlife. Agricultural lands can also be a source of negative environmental impacts (e.g., water pollution from nutrients, eroded soils, and pesticides).


A dynamic association of crops, pastures, livestock, other flora and fauna, atmosphere, soils, and water. Agroecosystems are contained within larger landscapes that include uncultivated land, drainage networks, rural communities, and wildlife.

Agronomic Systems

See Agricultural Lands.


In recent years, the term "airshed" has been used to describe those areas where significant portions of emissions result in deposition of the various air pollutants to a region. Many types of air pollution are carried by the wind from state to state, and are harmful to people and the environment, even in rural areas.


Simple rootless plants that grow in bodies of water (e.g., estuaries) at rates in relative proportion to the amounts of nutrients (e.g., nitrogen and phosphorus) available in the water.

Algae - photo
Photo courtesy of USDA NRCS.

Algae in a pond.

Alkaline Soil

Soil with a pH above 7.0.


The total measurable bases (OH, HCO3, CO3) in a volume of water; a measure of a material's capacity to neutralize acids; pH > 7.


An area of sand, clay or other similar material that has been gradually deposited by moving water.

Alluvial Channel

See Alluvial Stream.

Alluvial Deposits

Detrital material which is transported by a river and deposited - usually temporarily - at points along the flood plain of a river. Commonly composed of sands and gravels.

Alluvial Fan

Deposits of rock and soil that have eroded from mountainsides and accumulated on valley floors in a fan-shaped pattern and which occur mainly in dry mountainous areas. Erosion patterns and stream beds often shift erratically on an alluvial fan.

Alluvial fan - photo
Photo courtesy of USDA NRCS.

Photo of alluvial fan.

Alluvial Fan Flooding

Flooding occurring on the surface of an Alluvial Fan or similar landform which originates at the apex and is characterized by high-velocity flows: active processes of erosion, sediment transport, deposition, and unpredictable flow paths.

Alluvial Sediments

Relating to mud and/or sand deposited by flowing water. Alluvial deposits may occur after a heavy rain storm.

Alluvial Stream

A stream that has formed its channel by the process of aggradation. The sediment in the stream is similar to the material in the bed and banks.


A general term for all material (clay, silt, sand, gravel, or similar unconsolidated material) deposited or in transit by streams, including gravel, sand, silt, clay, and all variations and mixtures of these materials.

Ambient Water-Quality Criteria for the Protection of Aquatic Organisms

Non-enforceable guidelines for short-term (acute) and long-term (chronic) exposures to some pesticides. These guidelines provide the basis for state standards.

American Public Works Association (APWA)

Is an international educational and professional association of public agencies, private sector companies, and individuals dedicated to providing high quality public works goods and services. APWA is the largest and oldest organization of its kind in the world, with headquarters in Kansas City, MO., an office in Washington D.C., and 67 chapters throughout North America. APWA provides a forum in which public works professionals can exchange ideas, improve professional competency, increase the performance of agencies and companies, and bring important public works-related topics to public attention in local, state and federal arenas. APWA has 26,000 members.

American Water Resources Association (AWRA)

Founded in 1964, the American Water Resources Association is a non-profit professional association dedicated to the advancement of men and women in water resources management, research, and education. AWRA's membership is multidisciplinary; its diversity is its hallmark. It is the professional home of a wide variety of water resources experts including engineers, educators, foresters, biologists, ecologists, geographers, managers, regulators, hydrologists and attorneys.

Anadromous Fish

Fish that spend their adult lives in the sea but swim upriver into fresh water to spawn (e.g., salmon, striped bass, American shad).

Angle of Repose

Angle between the horizontal and the maximum slope that soil assumes bynatural process.


Abbreviation for area not included.

Annual Flood

The highest peak discharge of a stream in a Water Year.

Annual Flood Series

A list of annual floods for a given period of time.

Annual Plant

A plant that completes its life cycle and dies in one year or less.

Anoxic (Anoxia)

A condition where very little or no oxygen is present in the water body.


Originating from man, not naturally occurring.

Anthropogenic Cover

Land cover associated with human activities, such as agricultural fields, rock quarries, and urban areas. Literally, "land cover created by humans."

Antiscour Protection Apron

An apron in front of a revetment to help protect the revetment against underscour.

Anti-seep Collar

A device constructed around a pipe or other conduit and placed through a dam, levee, or dike for the purpose of reducing seepage losses and piping failures.


A floor or lining to protect a surface from erosion.


Water based liquid.


A geologic formation or structure that transmits water in sufficient quantity to supply the needs for a water development; usually saturated sands, gravel, fractures, and cavernous rock. The term "water-bearing" is sometimes used synonymously with aquifer when a stratum furnished water for a special use. A water-bearing soil layer, under the surface of the earth. This zone is often the source of drinking or irrigation water. Aquifers can be classified as confined or unconfined.


Underground geological formation that is slightly permeable and yields insignificant amounts of water when compared to an aquifer.

Area Flooded

Area of a floodplain that is flooded in a specific stream reach, watershed, or river basin; may be for a single flood event, but is usually expressed as an average, annual value based on the sum of areas from all individual flood events over a long period of time, such as 50 to 100 years, and adjusted to an average value.

Area of Shallow Flooding

Designated Flood Zones AO and AH on a community's Flood Insurance Rate Map (FIRM) with a one percent or greater annual chance of flooding to an average depth of one to three feet where a clearly defined channel does not exist, where the path of flooding is unpredictable and where velocity flow may be evident. Such flooding is characterized by ponding or sheet flow. Also referred to as Sheet Flow Area.

Area Of Special Flood-Related Erosion Hazard

The land within a community which is most likely to be subject to experience flood-related erosion losses. The area may be designated as Zone E on the Flood Insurance Rate Map (FIRM). See Flood Zones.


To protect; protective covering; shield. To use other hardened materials to reinforce an earthen levee.

Artesian Aquifer

An aquifer that contains water under pressure as a result of hydrostatic head. For artesian conditions to exist, an aquifer must be overlain by a confining material and receive a supply of water. The free water surface stands at a higher elevation than the top of the confining layer, thus if the aquifer is tapped by a well, the water in the well will raise above the level of the aquifer.

Artesian Well

Groundwater that is able to flow to the Earth's surface under its own pressure.

Artificial Obstruction

Artificial obstruction shall mean any obstruction which is not a natural obstruction.


Interpretation and evaluation of results for the purpose of answering policy-relevant questions about ecological resources, including: 1) determination of the fraction of the population that meets a socially defined value and 2) association among indicators of ecological condition and stressors. See Ecological Risk Assessment.

Assessment Endpoints

Detailed expressions of those values that have been identified as environmental or societal values associated with forests studied under the Forest Health Monitoring Program (FHP). See Ecological Risk Assessment.

Association of State Dam Safety Officials (ASDSO)

A national non-profit organization of state and federal dam safety regulators, dam owners and operators, engineering consultants, manufacturers and suppliers, academia, contractors and others interested in dams safety. See www.damsafety.org.

Association of State Floodplain Managers (ASFPM)

An organization of professionals involved in floodplain management, flood hazard mitigation, the National Flood Insurance Program, and flood preparedness, warning and recovery. ASFPM has become a respected voice in floodplain management practice and policy in the United States because it represents the flood hazard specialists of local, state and federal government, the research community, the insurance industry, and the fields of engineering, hydrologic forecasting, emergency response, water resources, and others. See www.floods.org.


The blanket of gases, vapor, and aerosol particles encasing the Earth.

Atmospheric Deposition

The flux (flow) of chemicals and materials from the atmosphere to the Earth's surface. Depending on the chemical or material, "dry" deposition (e.g., by particles) can be less than, equal to, or greater than "wet" deposition (e.g., precipitation).


The process by which a compound is reduced in concentration over time, through absorption, adsorption, degradation, dilution, and/or transformation. An also be the decrease with distance of sight caused by attenuation of light by particulate pollution.

Available Nutrient

That portion of any element or compound in the soil that readily can be absorbed and assimilated by growing plants.

Available Soil Moisture

The portion of water in a soil that can be readily absorbed by plant roots. It is the amount of water released between in situ field capacity and the permanent wilting point.

Average Annual Flood Damages

The weighted average of all flood damages that would be expected to occur yearly under specified economic conditions and development. Such damages are computed on the basis of the expectancy in any one year of the amounts of damage that would result from floods throughout the full range of potential magnitude.

Average Annual Flow

The rate at which water flows through a channel, determined by averaging daily measurements of the flow during one entire year.

Numeric | A | B | C | D | E | F | G | H | I | J | K
L | M | N | O | P | Q | R | S | T | U | V | W | XYZ

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